How to bake multiple materials on a single texture
Cube has attached three separate Shaders (materials) - Pink, Green, White ...

To bake them into one texture ... add Texture node with new (empty) image and keep node selected. Do the same for other materials (like here for Pink and White). Just copy&amp;paste node and keep selected.

In case you want bake only color pass, select Diffuse and uncheck Direct and Indirect Light.